Sensor

The sensor includes a filter member including cells that trap PM in exhaust gas; electrode members arranged to face each other with the cell interposed and forming a capacitor, an electric heater that executes, when an amount of PM has accumulated in the cells, filter regeneration of heating the cells to combust and remove the PM, a storage unit that stores a reference reduction amount, which is an electrostatic capacitance reduction amount between the electrode members in a case where the filter regeneration is executed in a state where the PM is not flowing into the filter member, and estimation units that estimate a PM amount based on (a) an actual electrostatic capacitance change amount between the electrode members during a regeneration interval period, and (b) a difference between an actual electrostatic capacitance reduction amount between the electrode members and the reference reduction amount during a filter regeneration period.